Nano-crystalline core and nano-crystalline shell pairing having group I-III-VI material nano-crystalline core

ABSTRACT

Nano-crystalline core and nano-crystalline shell pairings having group I-III-VI material nano-crystalline cores, and methods of fabricating nano-crystalline core and nano-crystalline shell pairings having group I-III-VI material nano-crystalline cores, are described. In an example, a semiconductor structure includes a nano-crystalline core composed of a group I-III-VI semiconductor material. A nano-crystalline shell composed of a second, different, semiconductor material at least partially surrounds the nano-crystalline core. In one specific example, the nano-crystalline core/nano-crystalline shell pairing has a photoluminescence quantum yield (PLQY) of greater than 60%. In another specific example, the nano-crystalline core/nano-crystalline shell pairing is a Type I hetero-structure.

TECHNICAL FIELD

Embodiments of the present invention are in the field of quantum dotsand, in particular, nano-crystalline core and nano-crystalline shellpairings having group I-III-VI material nano-crystalline cores.

BACKGROUND

Quantum dots having a high photoluminescence quantum yield (PLQY) may beapplicable as down-converting materials in down-convertingnano-composites used in solid state lighting applications.Down-converting materials are used to improve the performance,efficiency and color choice in lighting applications, particularly lightemitting diodes (LEDs). In such applications, quantum dots absorb lightof a particular first (available or selected) wavelength, usually blue,and then emit light at a second wavelength, usually red or green.

SUMMARY

Embodiments of the present invention include nano-crystalline core andnano-crystalline shell pairings having group I-III-VI materialnano-crystalline cores and methods of fabricating nano-crystalline coreand nano-crystalline shell pairings having group I-III-VI materialnano-crystalline cores.

In an embodiment, a semiconductor structure includes a nano-crystallinecore composed of a group I-III-VI semiconductor material. Anano-crystalline shell composed of a second, different, semiconductormaterial at least partially surrounds the nano-crystalline core. Thenano-crystalline core/nano-crystalline shell pairing has aphotoluminescence quantum yield (PLQY) of greater than 60%.

In another embodiment, a semiconductor structure includes anano-crystalline core composed of a group I-III-VI semiconductormaterial. A nano-crystalline shell composed of a second, different,semiconductor material at least partially surrounds the nano-crystallinecore. The nano-crystalline core/nano-crystalline shell pairing is a TypeI hetero-structure.

In another embodiment, a composite includes a matrix material and aplurality of semiconductor structures embedded in the matrix material.Each semiconductor structure includes a nano-crystalline core composedof a group I-III-VI semiconductor material and a nano-crystalline shellcomposed of a second, different, semiconductor material at leastpartially surrounding the nano-crystalline core. The nano-crystallinecore/nano-crystalline shell pairing has a photoluminescence quantumyield (PLQY) of greater than 60%. Each semiconductor structure furtherincludes an amorphous insulator coating surrounding and encapsulatingthe nano-crystalline core/nano-crystalline shell pairing.

In another embodiment, a composite includes a matrix material and aplurality of semiconductor structures embedded in the matrix material.Each semiconductor structure includes a nano-crystalline core composedof a group I-III-VI semiconductor material and a nano-crystalline shellcomposed of a second, different, semiconductor material at leastpartially surrounding the nano-crystalline core. The nano-crystallinecore/nano-crystalline shell pairing is a Type I hetero-structure. Eachsemiconductor structure further includes an amorphous insulator coatingsurrounding and encapsulating the nano-crystalline core/nano-crystallineshell pairing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of bandgaps of group II-VI and group I-III-VI directgap semiconductors, plotted relative to the vacuum level, in accordancewith an embodiment of the present invention.

FIG. 2 is a schematic illustrating a Type I core/shell hetero-structureand principal of operation, in accordance with an embodiment of thepresent invention.

FIG. 3 illustrates an axial cross-sectional view (A) and an electronicstructure diagram (B) of a graded copper indium sulfide (CIS)/AgGaS₂(AGS) nano-particle, in accordance with an embodiment of the presentinvention.

FIG. 4 is a plot of lattice constants (in Angstroms) of group I-III-VIand II-VI materials plotted along both X- and Y-axes, with the diagonalline representing same materials (0% mismatch), in accordance with anembodiment of the present invention.

FIG. 5 illustrates a schematic of a cross-sectional view of a quantumdot, in accordance with an embodiment of the present invention.

FIG. 6 illustrates a cross-sectional view of a semiconductor structurehaving a nano-crystalline core and nano-crystalline shell pairing withone compositional transition layer, in accordance with an embodiment ofthe present invention.

FIG. 7 illustrates a cross-sectional view of a semiconductor structurehaving a nano-crystalline core/nano-crystalline shell/nano-crystallineouter shell combination with two compositional transition layers, inaccordance with an embodiment of the present invention.

FIG. 8 illustrates a cross-sectional view of a semiconductor structurehaving a nano-crystalline core/nano-crystalline shell/nano-crystallineouter shell combination with one compositional transition layer, inaccordance with an embodiment of the present invention.

FIG. 9 illustrates operations in a reverse micelle approach to coating asemiconductor structure, in accordance with an embodiment of the presentinvention.

FIGS. 10A-10C illustrate schematic representations of possible compositecompositions for quantum dot integration, in accordance with anembodiment of the present invention.

FIG. 11 illustrates a lighting device that includes a blue LED with alayer having a composition with a dispersion of quantum dots therein, inaccordance with an embodiment of the present invention.

FIG. 12 illustrates a cross-sectional view of a lighting device with alayer having a composition with a dispersion of quantum dots therein, inaccordance with an embodiment of the present invention.

FIG. 13 illustrates a cross-sectional view of a lighting device with alayer having a composition with a dispersion of quantum dots therein, inaccordance with another embodiment of the present invention.

FIG. 14 illustrates a cross-sectional view of a lighting device with alayer having a composition with a dispersion of quantum dots therein, inaccordance with another embodiment of the present invention.

FIG. 15 illustrates a cross-sectional view of a lighting device with alayer having a composition with a dispersion of quantum dots therein, inaccordance with another embodiment of the present invention.

FIGS. 16A-16C illustrate cross-sectional views of various configurationsfor lighting devices with a layer having a composition with a dispersionof quantum dots therein, in accordance with another embodiment of thepresent invention.

FIG. 17 is a transmission electron microscope image of copper indiumsulfide (CIS) nano-particles of approximately 3 nanometers in size, witha scale bar of 5 nanometers, in accordance with an embodiment of thepresent invention.

FIG. 18 is a schematic showing a general growth method for silvergallium sulfide (AGS) nano-particles, in accordance with an embodimentof the present invention.

FIG. 19 is an X-ray diffraction (XRD) plot of intensity (counts) as afunction of Two-Theta (degrees) for a sample of silver gallium sulfide(AGS) nano-particles fabricated according to methods described herein,in accordance with an embodiment of the present invention.

FIG. 20 is a fluorescence spectrum of silver gallium sulfide (AGS)nano-particles grown with a 1:2 Ag:Ga precursor ratio, in accordancewith an embodiment of the present invention.

FIG. 21 is a fluorescence spectrum of silver gallium sulfide (AGS)nano-particles grown with a 1:4 Ag:Ga precursor ratio, in accordancewith an embodiment of the present invention.

DETAILED DESCRIPTION

Nano-crystalline core and nano-crystalline shell pairings having groupI-III-VI material nano-crystalline cores, and methods of fabricatingnano-crystalline core and nano-crystalline shell pairings having groupI-III-VI material nano-crystalline cores, are described herein. In thefollowing description, numerous specific details are set forth, such asspecific quantum dot geometries and efficiencies, in order to provide athorough understanding of embodiments of the present invention. It willbe apparent to one skilled in the art that embodiments of the presentinvention may be practiced without these specific details. In otherinstances, well-known related apparatuses, such as the host of varietiesof applicable light emitting diodes (LEDs) and apparatuses containingLEDs, are not described in detail in order to not unnecessarily obscureembodiments of the present invention. Furthermore, it is to beunderstood that the various embodiments shown in the figures areillustrative representations and are not necessarily drawn to scale.

One or more embodiments described herein are directed to improvementsfor quantum dot (QD) performance by fabrication of semiconductorhetero-structures having nano-crystalline cores composed of groupI-III-VI materials. Applications of such semiconductor hetero-structurescan include uses in light emitting diode (LED) applications,photovoltaics, sensing, photonics, and biotechnology, as examples. Oneor more embodiments are directed to the fabrication of cadmium (Cd)-freequantum dots (QDs).

To provide context, one or more embodiments is directed to a system ofCd-free materials which serve as high efficiency downconverting quantumdot materials for applications which benefit from the high conversionefficiency of quantum dots, but which cannot tolerate Cd-containingmaterials. In particular, the materials can serve as downconverters forlighting applications, where the material acts in place of, or togetherwith, a convention phosphor to convert high energy light to lower energylight, and the combination of colors appears white to the observer. PastCd-free work has focused on copper indium sulfide (CIS) or copper indiumgallium sulfide (CIGS) emitter systems in combination with a protectiveZnS shell. Some specific embodiments described herein instead focus onthe use of Ag-based materials, particularly the I-III-VI systems AgGaS₂and AgGaSe₂. These materials have the advantage that they can be tunednot just by size, but also by the stoichiometry of the material. Aswritten, these materials have bandgaps which are suitable for the shellmaterial in a core/shell QD system. However, the inventors have foundthat lower energy emission can be obtained by varying the stoichiometryof the material system. Therefore, in an embodiment, such materials aresuitable both as core and shell materials, in combination with eitherother I-III-VI materials or with II-VI materials.

FIG. 1 is a plot 100 of bandgaps of group II-VI and group I-III-VIdirect gap semiconductors, plotted relative to the vacuum level, inaccordance with an embodiment of the present invention. Region 102 ofplot 100 includes direct band gaps for AgGaS₂ and AgGaSe₂ group I-III-VImaterials. For the purposes of high performance (high photoluminescentquantum yield and high stability) a typical quantum dot system iscomposed of a core material overcoated epitaxially with a shellmaterial. For the purposes of very high luminescent quantum yield, thecore bandgap should be contained (nested) within the shell bandgap. Thisconfiguration electronically shields the excited state from surfacetraps while promoting overlap between the electron and hole, whichincreases the probability for radiative emission from the excited state.Nesting of the bandgap of the core material inside the bandgap of theshell material is known as a “Type 1” electronic structure.

Quantum dots based on II-VI semiconductors have historically been themost widely studied, and the synthetic process to control emissionplacement and quantum efficiency is very well understood. However,studies of non-Cd based systems have commenced and PLQYs between 30% and80% have been published, primarily driven by the need in solar andbiological applications for Cd-free materials. In choosing a Cd-freesystem of materials for solid state lighting (SSL) applications whichmimics the benefits of Cd-based systems already demonstrated, however,the criteria are somewhat different.

In a first aspect, in an embodiment, a core emitter material is selectedor designed to have a direct gap semiconductor with a bulk bandgapideally in the 1-2 eV range. This factor is considered since direct gapsemiconductors have the appropriate quantum efficiencies and excitonlifetimes, and because the emission of the QD can only shift blue fromthe bulk value when tuning by size of the emitter material. As acomparison, cadmium selenide (CdSe) has a somewhat ideal bandgap of 1.74eV (712 nanometers) which allows tuning of the quantum dot (QD) emissionacross the visible spectrum. However, some materials such as theI-III-VI materials described herein are susceptible to emission tuningboth by size and stoichiometry of the material.

In a second aspect, in an embodiment, an appropriate core/shell pairingis selected or designed to provide a Type I system, promoting radiativerecombination in the core (emitter material). Additionally, a Type 1pairing can allow for absorption and emission to be tuned separately.However, the bulk value of the shell bandgap should not be so far intothe UV that the blue excitation light is not well-absorbed.

FIG. 2 is a schematic 200 illustrating a Type I core 202/shell 204hetero-structure and principal of operation, in accordance with anembodiment of the present invention. At stage 1, absorption of ahigh-energy photon by the shell 204 material and generation of anelectron (e⁻)-hole (h⁺) pair occurs. At stage 2, relaxation into alower-energy core state occurs. At stage 3, emission via radiativerecombination of the electron-hole pair in the core 202 material occurs.It is to be understood that emission from the core 202 travels throughthe shell material 204, as depicted in FIG. 2.

In a third aspect, in an embodiment, a relatively small lattice mismatchbetween the core and shell is used. As a Cd-based example, CdSe and CdShave an approximately 4% lattice mismatch. Such small mismatch allowsfor a very thick and/or asymmetric shell to be grown on the core, andalso allows tuning of the relative absorption of the core and shell. Itmay be preferable to grow a very large shell so that the shell is thedominant absorber, and the core is the dominant emitter.

In a fourth aspect, in an embodiment, core/shell pairings are selectedor designed to provide an inherently stable system for temperatures upto approximately 200 degrees Celsius, or at least to provide suitabilityfor integration into an LED-based luminaire. For example, QDs which aredoped can have a broad emission line width likely due to the variationsin the position of the dopant atoms, rendering them a poor candidate fora red phosphor replacement material. Additionally, dopants can easily be“annealed” out from the core of the QD to the surface, making them muchmore susceptible to non-radiative recombination.

To address and accommodate one or more of the above aspects, one or moreembodiments described herein involve fabrication of a semiconductorhetero-structure. The semiconductor hetero-structure has anano-crystalline core composed of a group I-III-VI semiconductormaterial. A nano-crystalline shell composed of a second, different,semiconductor material at least partially surrounds the nano-crystallinecore. For example, the nano-crystalline shell may be composed of adifferent group I-Ill-VI semiconductor material or of a group II-VIsemiconductor material.

In one such embodiment, the above described nano-crystallinecore/nano-crystalline shell pairing has a photoluminescence quantumyield (PLQY) of greater than approximately 60%. In another, or same,such embodiment, the nano-crystalline core/nano-crystalline shellpairing provides a Type I hetero-structure. By contrast, a report by Liet al, Chem. Mater. 2009, v. 21, no. 12, pgs 2422-2429 referred to aType I hetero-structure with a CuInS core. However, the system of Li etal. showed emission data based on bulk emission measurements. It is thepresent inventors' contention that the seemingly low PLQY reportedtherein suggests a fully alloyed structure without a distinct groupI-III-VI material core. As such, one or more embodiments describedherein are directed to hetero-structure systems having distinct groupI-III-VI material cores.

In an exemplary embodiment, a sphere or rod-shaped core/shell quantumdot is fabricated to have a sharp compositional interface between thecore and shell or a graded/alloyed interface between core and shell.FIG. 3 illustrates an axial cross-sectional view (A) of a graded copperindium sulfide (CIS)/AgGaS₂ (AGS) nano-particle 300, in accordance withan embodiment of the present invention. Referring to FIG. 3, an alloyregion 306 is included between the core 302 and shell 304 of 300. Asshown in part (B) of FIG. 3, in one embodiment, the nano-particle 300demonstrates type I hetero-structure behavior, with excitonspreferentially recombining in the core 302 of the nano-crystal 300 dueto the smaller, nested bandgap of CIS (approximately 1.7 eV) versus AGS(approximately 2.65 eV). Optionally, additional layers of material maybe added, including additional epitaxial layers or amorphous inorganicand organic layers. Other suitable embodiments are described below.

In an embodiment, systems described herein include a nano-crystallinecore emitter having a direct, bulk band gap approximately in the rangeof 1-2.5 eV. Exemplary cores include a group I-III-VI semiconductormaterial based on silver gallium sulfide having a stoichiometry ofapproximately AgGaS₂. In one such embodiment, the nano-crystalline corehas a peak emission approximately in the range of 475-575 nanometers.

In one or more embodiments, the nano-crystalline core andnano-crystalline shell pairings described herein have a lattice mismatchof equal to or less than approximately 10%. In some embodiments, lessthan approximately 6% mismatch is preferable, but up to approximately10% can be workable. In particular embodiments, the mismatch is lessthan approximately 4% mismatch, as seen in successful Cd-based systems.FIG. 4 is a plot 400 of lattice constants (in Angstroms) of groupI-III-VI and II-VI materials plotted along both X- and Y-axes, with thediagonal line representing same materials (0% mismatch), in accordancewith an embodiment of the present invention. Referring to FIG. 4,lattice mismatches for corresponding material combinations are provided.The lattice mismatch values can be used to determine suitablecombinations of core/shell materials for high performancehetero-structure quantum dot fabrication.

As mentioned briefly above, one or more embodiments described herein isdirected to a hetero-structure core/shell pairing that is cadmium-free.For example, with reference to the above described nano-crystalline coreand nano-crystalline shell pairings, in an embodiment, the first (core)material is a group I-III-VI semiconductor material. In one suchembodiment, the second (shell) semiconductor material is a second groupI-III-VI material. For example, a suitable I-III-VI/I-III-VI core/shellpairing can include, but is not limited to, copper indium sulfide(CIS)/silver gallium sulfide (AgGaS₂), copper indium selenide(CISe)/AgGaS₂, copper gallium selenide (CuGaSe₂)/copper gallium sulfide(CuGaS₂), or CuGaSe₂/AgGaS₂. In another such embodiment, the second(shell) semiconductor material is a group II-VI material. For example, asuitable I-III-VI/II-VI core/shell pairing can include, but is notlimited to, copper indium sulfide (CIS)/zinc selenide (ZnSe), CIS/zincsulfide (ZnS), copper indium selenide (CISe)/ZnSe, CISe/ZnS, coppergallium selenide (CuGaSe₂)/ZnSe, CuGaSe₂/ZnS, silver gallium sulfide(AgGaS₂)/ZnS, AgGaS₂/ZnSe, or silver gallium selenide (AgGaSe₂)/ZnS,AgGaSe₂/ZnSe.

In an embodiment, the semiconductor hetero-structure further includes anano-crystalline outer shell composed of a third semiconductor materialdifferent from the core and shell semiconductor materials. The thirdsemiconductor material at least partially surrounding thenano-crystalline shell and, in one embodiment, the nano-crystallineouter shell completely surrounds the nano-crystalline shell. In aparticular embodiment, the second (shell) semiconductor material onesuch as, but not limited to, zinc selenide (ZnSe), silver galliumsulfide (AgGaS₂) or copper gallium sulfide (CuGaS₂), and the third(outer shell) semiconductor material is zinc sulfide (ZnS).

Thus, embodiments described herein are directed to nano-particles basedon semiconductor hetero-structures having nano-crystalline corescomposed of group I-III-VI materials, such as hetero-structure-basedquantum dots. Such hetero-structures may have specific geometriessuitable for performance optimization, e.g., for high performancequantum dot behavior. In an example, several factors may be intertwinedfor establishing an optimized geometry for a quantum dot having anano-crystalline core and nano-crystalline shell pairing. As areference, FIG. 5 illustrates a schematic of a cross-sectional view of aquantum dot, in accordance with an embodiment of the present invention.

Referring to FIG. 5, a semiconductor structure (e.g., a quantum dotstructure) 500 includes a nano-crystalline core 502 surrounded by anano-crystalline shell 504. The nano-crystalline core 502 has a lengthaxis (_(aCORE)), a width axis (_(bCORE)) and a depth axis (_(cCORE)),the depth axis provided into and out of the plane shown in FIG. 5.Likewise, the nano-crystalline shell 504 has a length axis (_(aSHELL)),width axis (_(bSHELL)) and a depth axis (_(cSHELL)), the depth axisprovided into and out of the plane shown in FIG. 5. The nano-crystallinecore 502 has a center 503 and the nano-crystalline shell 504 has acenter 505. The nano-crystalline shell 504 surrounds thenano-crystalline core 502 in the b-axis direction by an amount 506, asis also depicted in FIG. 5.

The following are attributes of a quantum dot that may be tuned foroptimization, with reference to the parameters provided in FIG. 5, inaccordance with embodiments of the present invention. Nano-crystallinecore 502 diameter (a, b or c) and aspect ratio (e.g., a/b) can becontrolled for rough tuning for emission wavelength (a higher value foreither providing increasingly red emission). A smaller overallnano-crystalline core provides a greater surface to volume ratio. Thewidth of the nano-crystalline shell along 506 may be tuned for yieldoptimization and quantum confinement providing approaches to controlred-shifting and mitigation of surface effects. However, strainconsiderations must be accounted for when optimizing the value ofthickness 506. The length (a_(SHELL)) of the shell is tunable to providelonger radiative decay times as well as increased light absorption. Theoverall aspect ratio of the structure 500 (e.g., the greater ofa_(SHELL)/b_(SHELL) and a_(SHELL)/c_(SHELL)) may be tuned to directlyimpact PLQY. Meanwhile, overall surface/volume ratio for 500 may be keptrelatively smaller to provide lower surface defects, provide higherphotoluminescence, and limit self-absorption. Referring again to FIG. 5,the shell/core interface 508 may be tailored to avoid dislocations andstrain sites. In one such embodiment, a high quality interface isobtained during nano-particle fabrication by tailoring one or more ofprecursor injection temperature and mixing parameters, the use ofsurfactants, and control of the reactivity of precursors.

In accordance with an embodiment of the present invention, a high PLQYquantum dot is based on a core/shell pairing using an anisotropic core.With reference again to FIG. 5, an anisotropic core is a core having oneof the axes a_(CORE), b_(CORE) or c_(CORE) different from one or both ofthe remaining axes. An aspect ratio of such an anisotropic core isdetermined by the longest of the axes a_(CORE), b_(CORE) or c_(CORE)divided by the shortest of the axes a_(CORE), b_(CORE) or c_(CORE) toprovide a number greater than 1 (an isotropic core has an aspect ratioof 1). It is to be understood that the outer surface of an anisotropiccore may have rounded or curved edges (e.g., as in an ellipsoid) or maybe faceted (e.g., as in a stretched or elongated tetragonal or hexagonalprism) to provide an aspect ratio of greater than 1 (note that a sphere,a tetragonal prism, and a hexagonal prism are all considered to have anaspect ratio of 1 in keeping with embodiments of the present invention).

A workable range of aspect ratio for an anisotropic nano-crystallinecore for a quantum dot may be selected for maximization of PLQY. Forexample, a core that is essentially isotropic may not provide advantagesfor increasing PLQY, while a core with too great an aspect ratio (e.g.,2 or greater) may present challenges synthetically and geometricallywhen forming a surrounding shell. Furthermore, embedding the core in ashell composed of a material different than the core may also be usedenhance PLQY of a resulting quantum dot.

Accordingly, in an embodiment, a semiconductor structure includes ananisotropic nano-crystalline core composed of a first semiconductormaterial, e.g., a group I-III-VI material, and having an aspect ratiobetween, but not including, 1.0 and 2.0. The semiconductor structurealso includes a nano-crystalline shell composed of a second, different,semiconductor material at least partially surrounding the anisotropicnano-crystalline core. In one such embodiment, the aspect ratio of theanisotropic nano-crystalline core is approximately in the range of1.01-1.2 and, in a particular embodiment, is approximately in the rangeof 1.1-1.2. In the case of rounded edges, then, the nano-crystallinecore may be substantially, but not perfectly, spherical. However, thenano-crystalline core may instead be faceted. In an embodiment, theanisotropic nano-crystalline core is disposed in an asymmetricorientation with respect to the nano-crystalline shell, as described ingreater detail in the example below.

Another consideration for maximization of PLQY in a quantum dotstructure is to provide an asymmetric orientation of the core within asurrounding shell. For example, referring again to FIG. 5, the center503 of the core 502 may be misaligned with (e.g., have a differentspatial point than) the center 505 of the shell 504. In an embodiment, asemiconductor structure includes an anisotropic nano-crystalline corecomposed of a first semiconductor material. The semiconductor structurealso includes a nano-crystalline shell composed of a second, different,semiconductor material at least partially surrounding the anisotropicnano-crystalline core. The anisotropic nano-crystalline core is disposedin an asymmetric orientation with respect to the nano-crystalline shell.In one such embodiment, the nano-crystalline shell has a long axis(e.g., a_(SHELL)), and the anisotropic nano-crystalline core is disposedoff-center along the long axis. In another such embodiment, thenano-crystalline shell has a short axis (e.g., b_(SHELL)), and theanisotropic nano-crystalline core is disposed off-center along the shortaxis. In yet another embodiment, however, the nano-crystalline shell hasa long axis (e.g., a_(SHELL)) and a short axis (e.g., b_(SHELL)), andthe anisotropic nano-crystalline core is disposed off-center along boththe long and short axes.

With reference to the above described nano-crystalline core andnano-crystalline shell pairings, in an embodiment, the nano-crystallineshell completely surrounds the anisotropic nano-crystalline core. In analternative embodiment, however, the nano-crystalline shell onlypartially surrounds the anisotropic nano-crystalline core, exposing aportion of the anisotropic nano-crystalline core, e.g., as in a tetrapodgeometry or arrangement. In an embodiment, the nano-crystalline shell isan anisotropic nano-crystalline shell, such as a nano-rod, thatsurrounds the anisotropic nano-crystalline core at an interface betweenthe anisotropic nano-crystalline shell and the anisotropicnano-crystalline core. The anisotropic nano-crystalline shell passivatesor reduces trap states at the interface. The anisotropicnano-crystalline shell may also, or instead, deactivate trap states atthe interface.

With reference again to the above described nano-crystalline core andnano-crystalline shell pairings, in an embodiment, the semiconductorstructure (i.e., the core/shell pairing in total) has an aspect ratioapproximately in the range of 1.5-10 and, 3-6 in a particularembodiment. In an embodiment, the nano-crystalline shell has a long axisand a short axis. The long axis has a length approximately in the rangeof 5-40 nanometers. The short axis has a length approximately in therange of 1-5 nanometers greater than a diameter of the anisotropicnano-crystalline core parallel with the short axis of thenano-crystalline shell. In a specific such embodiment, the anisotropicnano-crystalline core has a diameter approximately in the range of 2-5nanometers. The thickness of the nano-crystalline shell on theanisotropic nano-crystalline core along a short axis of thenano-crystalline shell is approximately in the range of 1-5 nanometersof the second semiconductor material.

With reference again to the above described nano-crystalline core andnano-crystalline shell pairings, in an embodiment, the anisotropicnano-crystalline core and the nano-crystalline shell form a quantum dot.In one such embodiment, the quantum dot has a photoluminescence quantumyield (PLQY) of at least 60%. Emission from the quantum dot may bemostly, or entirely, from the nano-crystalline core. For example, in anembodiment, emission from the anisotropic nano-crystalline core is atleast approximately 75% of the total emission from the quantum dot. Anabsorption spectrum and an emission spectrum of the quantum dot may beessentially non-overlapping. For example, in an embodiment, anabsorbance ratio of the quantum dot based on absorbance at 400nanometers versus absorbance at an exciton peak for the quantum dot isapproximately in the range of 5-35.

In an embodiment, a quantum dot based on the above describednano-crystalline core and nano-crystalline shell pairings is adown-converting quantum dot. However, in an alternative embodiment, thequantum dot is an up-shifting quantum dot. In either case, a lightingapparatus may include a light emitting diode and a plurality of quantumdots such as those described above. The quantum dots may be appliedproximal to the LED and provide down-conversion or up-shifting of lightemitted from the LED. Thus, semiconductor structures according to thepresent invention may be advantageously used in solid state lighting.The visible spectrum includes light of different colors havingwavelengths between about 380 nm and about 780 nm that are visible tothe human eye. An LED will emit a UV or blue light which isdown-converted (or up-shifted) by semiconductor structures describedherein. Any suitable ratio of emission color from the semiconductorstructures may be used in devices of the present invention. LED devicesaccording to embodiments of the present invention may have incorporatedtherein sufficient quantity of semiconductor structures (e.g., quantumdots) described herein capable of down-converting any available bluelight to red, green, yellow, orange, blue, indigo, violet or othercolor. These structures may also be used to downconvert or upconvertlower energy light (green, yellow, etc) from LED devices, as long as theexcitation light produces emission from the structures.

The above described semiconductor hetero-structures, e.g., quantum dots,may be fabricated to further include one or more compositionaltransition layers between portions of the structures, e.g., between coreand shell portions. Inclusion of such a transition layer may reduce oreliminate any performance inefficiency associated with otherwise abruptjunctions between the different portions of the structures. For example,the inclusion of a compositional transition layer may be used tosuppress Auger recombination within a quantum dot structure. Augerrecombination events translate to energy from one exciton beingnon-radiatively transferred to another charge carrier. Suchrecombination in quantum dots typically occurs on sub-nanosecond timescales such that a very short multi-exciton lifetime indicatesnon-radiative recombination, while higher nanosecond bi-excitonlifetimes indicate radiative recombination. A radiative bi-exciton has alifetime approximately 2-4 times shorter than radiative single exciton.

More specifically, as is described in greater detail below inassociation with FIGS. 6-8, an optimal particle (e.g., quantum dotstructure) may have one or more of a high aspect ratio, a large volumerelative to other quantum dot hetero-structures, and graded or alloyedtransitions between different semiconductor materials. The graded oralloyed transitions can be used to render a compositional and structuraltransition from one component (such as a quantum dot core composed of agroup I-III-VI material) to another component (such as a quantum dotshell) a smooth function rather than a step function. In one embodiment,the terms “graded,” “gradient,” or “grading” are used to convey gradualtransitioning from one semiconductor to another. In one embodiment, theterms “alloy,” “alloyed,” or “alloying” are used to convey an entirevolume having a fixed intermediate composition. In more specificembodiments, core or seed volume is maximized relative to shell volumefor a given emission color. A graded or alloyed core/shell transitionlayer may be included between the two volumes.

In a first example, FIG. 6 illustrates a cross-sectional view of asemiconductor structure having a nano-crystalline core andnano-crystalline shell pairing with one compositional transition layer,in accordance with an embodiment of the present invention.

Referring to FIG. 6, a semiconductor structure 600 includes a groupI-III-VI material (first material) nano-crystalline core 602. Anano-crystalline shell 604 composed of a second, different,semiconductor material at least partially surrounds the nano-crystallinecore 602. A compositional transition layer 610 is disposed between, andin contact with, the nano-crystalline core 602 and nano-crystallineshell 604. The compositional transition layer 610 has a compositionintermediate to the first and second semiconductor materials.

In an embodiment, the compositional transition layer 610 is an alloyedlayer composed of a mixture of the first and second semiconductormaterials. In another embodiment, the compositional transition layer 610is a graded layer composed of a compositional gradient of the firstsemiconductor material proximate to the nano-crystalline core 602through to the second semiconductor material proximate to thenano-crystalline shell 604. In either case, in a specific embodiment,the compositional transition layer 610 has a thickness approximately inthe range of 1.5-2 monolayers.

In accordance with an embodiment of the present invention, thecompositional transition layer 610 passivates or reduces trap stateswhere the nano-crystalline shell 604 surrounds the nano-crystalline core602. Exemplary embodiments of core and/or shell parameters include astructure 600 where the nano-crystalline core 602 is an anisotropicnano-crystalline core having an aspect ratio between, but not including,1.0 and 2.0 (in a specific embodiment, approximately in the range of1.01-1.2), and the nano-crystalline shell is an anisotropicnano-crystalline shell having an aspect ratio approximately in the rangeof 2-6.

In an embodiment, the nano-crystalline shell 604 completely surroundsthe nano-crystalline core 602, as depicted in FIG. 6. In an alternativeembodiment, however, the nano-crystalline shell 604 only partiallysurrounds the nano-crystalline core 602, exposing a portion of thenano-crystalline core 602. Furthermore, in either case, thenano-crystalline core 602 may be disposed in an asymmetric orientationwith respect to the nano-crystalline shell 604. In one or moreembodiments, semiconductor structures such as 600 are fabricated tofurther include a nano-crystalline outer shell 606 at least partiallysurrounding the nano-crystalline shell 604. The nano-crystalline outershell 606 may be composed of a third semiconductor material differentfrom the first and second semiconductor materials, i.e., different fromthe materials of the core 602 and shell 604. The nano-crystalline outershell 606 may completely surround the nano-crystalline shell 604 or mayonly partially surround the nano-crystalline shell 604, exposing aportion of the nano-crystalline shell 604.

For embodiments including a nano-crystalline outer shell, an additionalcompositional transition layer may be included. Thus, in a secondexample, FIG. 7 illustrates a cross-sectional view of a semiconductorstructure having a nano-crystalline core/nano-crystallineshell/nano-crystalline outer shell combination with two compositionaltransition layers, in accordance with an embodiment of the presentinvention.

Referring to FIG. 7, a semiconductor structure 700 includes the groupI-III-VI material nano-crystalline core 602, nano-crystalline shell 604,nano-crystalline outer shell 606 and compositional transition layer 610of structure 600. However, in addition, semiconductor structure 700includes a second compositional transition layer 712 disposed between,and in contact with, the nano-crystalline shell 604 and thenano-crystalline outer shell 606. The second compositional transitionlayer 712 has a composition intermediate to the second and thirdsemiconductor materials, i.e., intermediate to the semiconductormaterials of the shell 604 and outer shell 606.

In an embodiment, the second compositional transition layer 712 is analloyed layer composed of a mixture of the second and thirdsemiconductor materials. In another embodiment, the second compositionaltransition layer 712 is a graded layer composed of a compositionalgradient of the second semiconductor material proximate to thenano-crystalline shell 604 through to the third semiconductor materialproximate to the nano-crystalline outer shell 606. In either case, in aspecific embodiment, the second compositional transition layer 712 has athickness approximately in the range of 1.5-2 monolayers. In accordancewith an embodiment of the present invention, the second compositionaltransition layer 712 passivates or reduces trap states where thenano-crystalline outer shell 606 surrounds the nano-crystalline shell604.

For other embodiments including a nano-crystalline outer shell, an outercompositional transition layer may be included without including aninner compositional transition layer. Thus, in a third example, FIG. 8illustrates a cross-sectional view of a semiconductor structure having anano-crystalline core/nano-crystalline shell/nano-crystalline outershell combination with one compositional transition layer, in accordancewith an embodiment of the present invention.

Referring to FIG. 8, a semiconductor structure 800 includes the groupI-III-VI material nano-crystalline core 602, the nano-crystalline shell604, and nano-crystalline outer shell 606 of structure 600. In addition,the semiconductor structure 800 includes the compositional transitionlayer 712 of structure 700 disposed between, and in contact with, thenano-crystalline shell 604 and the nano-crystalline outer shell 606.However, structure 800 does not include the compositional transitionlayer 610 of structure 600, i.e., there is no compositional transitionlayer between the core 602 and shell 604.

Referring again to FIGS. 5-8, and as depicted in FIGS. 6-8, thestructures 500, 600, 700 and 800 may further include an insulatorcoating (e.g., shown as 608 in FIGS. 6-8) surrounding and encapsulatingthe nano-crystalline core/nano-crystalline shell pairing ornano-crystalline core/nano-crystalline shell/nano-crystalline outershell combination. In one such embodiment, the insulator coating iscomposed of an amorphous material such as, but not limited to, silica(SiO_(x)), titanium oxide (TiO_(x)), zirconium oxide (ZrO_(x)), alumina(AlO_(x)), or hafnia (HfO_(x)). In an embodiment, insulator-coatedstructures based on structures 500, 600, 700 and 800 are quantum dotstructures. For example, structures 500, 600, 700 and 800 may be used asa down-converting quantum dot or an up-shifting quantum dot.

The above described insulator coating may be formed to encapsulate aquantum dot using a reverse micelle process. For example, FIG. 9illustrates operations in a reverse micelle approach to coating asemiconductor structure, in accordance with an embodiment of the presentinvention. Referring to part A of FIG. 9, a quantum dot hetero-structure(QDH) 902 (e.g., a nano-crystalline core/shell pairing) has attachedthereto a plurality of TOPO ligands 904 and TOP ligands 906. Referringto part B, the plurality of TOPO ligands 904 and TOP ligands 906 areexchanged with a plurality of Si(OCH₃)₃(CH₂)₃NH₂ ligands 908. Thestructure of part B is then reacted with TEOS (Si(OEt)₄) and ammoniumhydroxide (NH₄OH) to form a silica coating 910 surrounding the QDH 902,as depicted in part C of FIG. 9.

In another aspect, a matrix including semiconductor hetero-structureshaving nano-crystalline cores composed of group I-III-VI materials isapplied to a lighting device to provide a layer having a dispersion ofthe semiconductor structures therein for inclusion in the lightingdevice. The matrices can include a dispersion of semiconductorstructures such as those described above in association with FIGS. 5-8.In a general embodiment, a composite includes a matrix material. Aplurality of semiconductor structures (e.g., quantum dot structureshaving a coated or non-coated core/shell pairing, such as the structuresdescribed above) is embedded in the matrix material. In an embodiment, alighting apparatus includes a light emitting diode and a compositecoating the light emitting diode. The composite may be formed byembedding quantum dots in a matrix material such as the matrix materialsdescribed below.

With reference to the above described composite, in an embodiment, eachof the plurality of semiconductor structures is cross-linked with,polarity bound by, or tethered to the matrix material. In an embodiment,each of the plurality of semiconductor structures is bound to the matrixmaterial by a covalent, dative, or ionic bond. By way of example, FIGS.10A-10C illustrate schematic representations of possible compositecompositions for quantum dot integration, in accordance with anembodiment of the present invention. Referring to FIG. 10A, anano-crystalline core 1002A and shell 1004A pairing is incorporated intoa polymer matrix 1006A by active cross-linking through multiple andinterchain binding to form a cross-linked composition 1008A. Referringto FIG. 10B, a nano-crystalline core 1002B and shell 1004B pairing isincorporated into a polymer matrix 1006B by polarity-based chemicalsimilarity and dissolution to form a polarity based composition 1008B.Referring to FIG. 10C, a nano-crystalline core 1002C and shell 1004Cpairing is incorporated into a polymer matrix 1006C by reactivetethering by sparse binding and chemical similarity to form a reactivetethering based composition 1008C.

With reference again to the above described composite, in an embodiment,one or more of the semiconductor structures further includes a couplingagent covalently bonded to an outer surface of the insulator layer. Forexample, in one such embodiment, the insulator layer includes or is alayer of silica (SiO_(x)), and the coupling agent is a silane couplingagent, e.g., having the formula X_(n)SiY_(4-n), where X is a functionalgroup capable of bonding with the matrix material and is one such as,but not limited to, hydroxyl, alkoxy, isocyanate, carboxyl, epoxy,amine, urea, vinyl, amide, aminoplast and silane, Y is a functionalgroup such as, but not limited to, hydroxyl, phenoxy, alkoxy, hydroxylether, silane or aminoplast, and n is 1, 2 or 3. In another embodiment,however, the coupling agent is one such as, but not limited to, atitanate coupling agent or a zirconate coupling agent. It is to beunderstood that the terms capping agent, capping ligand, ligand andcoupling agent may be used interchangeably as described above and,generally, may include an atom, molecule or other chemical entity ormoiety attached to or capable of being attached to a nano-particle.Attachment may be by dative bonding, covalent bonding, ionic bonding,Van der Waals forces or other force or bond.

In the case that a silica surface of a silica coated quantum dot ismodified using silane coupling agents having multiple functionalmoieties, coupling to the surface of the silica shell and coupling to amatrix material and/or other matrix additives may be enabled. Such anapproach provides uniform dispersion throughout the composite matrixusing as little effort (e.g., reaction energy) as possible. Strongerphysical and/or chemical bonding between the silica coated quantum dotsand the matrix resin occurs. Also, the silane coupling composition mustbe compatible with both the silica coated quantum dot, which isinorganic, and the polymer matrix, which may be organic. Without beingbound by any particular theory or principle, it is believed that thesilane coupling agent forms a bridge between the silica and the matrixresin when reactive functional groups on the silane coupling agentinteract with functional groups on the surface of the silica and/or thematrix resin. Because the functional groups involved are typically polarin nature, the coupling agent tends to be hydrophilic and readilydispersed in an aqueous size composition.

Matrix materials suitable for embodiments of the present invention maysatisfy the following criteria: they may be optically clear havingtransmission in the 400-700 nm range of greater than 90%, as measured ina UV-Vis spectrometer. The matrix material may have a high refractiveindex between about 1.0 and 2.0, preferably above 1.4 in the 400-700 nmrange. The matrix material may also have good adhesion to an LED surfaceif required and/or are sufficiently rigid for self-supportingapplications. And, the matrix material may able to maintain theirproperties over a large temperature range, for example −40° C. to 150°C. and over a long period of time (over 50,000 hours at a lightintensity typically 1-10 w/cm2 of 450 nm blue light).

Thus, with reference again to the above described composite, in anembodiment, the insulator layer is composed of a layer of silica(SiO_(x)), and the matrix material is composed of a siloxane copolymer.In another embodiment, the matrix material has a UV-Vis spectroscopytransmission of greater than 90% for light in the range of 400-700nanometers. In an embodiment, the matrix material has a refractive indexapproximately in the range of 1-2 for light in the range of 400-700nanometers. In an embodiment, the matrix material is thermally stable ina temperature range of −40-250 degrees Celsius. In an embodiment, thematrix material is composed of a polymer such as, but not limited to,polypropylene, polyethylene, polyesters, polyacetals, polyamides,polyacrylamides, polyimides, polyethers, polyvinylethers, polystyrenes,polyoxides, polycarbonates, polysiloxanes, polysulfones, polyanhydrides,polyamines, epoxies, polyacrylics, polyvinylesters, polyurethane, maleicresins, urea resins, melamine resins, phenol resins, furan resins,polymer blends, polymer alloys, or mixtures thereof. In one suchembodiment, the matrix material is composed of a polysiloxane such as,but not limited to, polydimethylsiloxane (PDMS),polymethylphenylsiloxane, polydiphenylsiloxane and polydiethylsiloxane.In an embodiment, the matrix material is composed of a siloxane such as,but not limited to, dimethylsiloxane or methylhydrogen siloxane.

Additionally, with reference again to the above described composite, inan embodiment, the plurality of semiconductor structures is embeddedhomogeneously in the matrix material. In an embodiment, the compositefurther includes a compounding agent embedded in the matrix material.The compounding agent is one such as, but not limited to, anantioxidant, a pigment, a dye, an antistatic agent, a filler, a flameretardant, an ultra-violet (UV) stabilizer, or an impact modifier. Inanother embodiment, the composite further includes a catalyst embeddedin the matrix material, the catalyst one such as, but not limited to, athiol catalyst or a platinum (Pt) catalyst.

Accordingly, in an embodiment, a method of fabrication includes forminga plurality of semiconductor hetero-structures embedded in a matrixmaterial (or embedding preformed semiconductor structures in a matrixmaterial). In one such embodiment, embedding the plurality ofsemiconductor structures in the matrix material includes cross-linking,reactive tethering, or ionic bonding the plurality of semiconductorstructures with the matrix material. In an embodiment, the methodfurther includes surface-functionalizing an insulator layer for thesemiconductor structures prior to embedding the plurality ofsemiconductor structures in the matrix material. In one such embodiment,the surface-functionalizing includes treating the insulator layer with asilane coupling agent. However, in an alternative embodiment, coatedsemiconductor structures are embedded in a matrix by using a ligand-freeinsulator layer.

In another embodiment, simple substitution at the surface of the silicacoated quantum dots is effective for stable integration withoutundesired additional viscosity and is suitable to produce alow-viscosity product such as a silicone gel. In one embodiment of thepresent invention a composite incorporates quantum dots which crosslinkwith the matrix through silane groups and which possess an adequatenumber of silane groups in order to form an elastic network. Inaddition, adequate adhesion to various substrates is enabled.Furthermore, silicone-based matrixes may be used. A structure of suchpolymers may be obtained which form microstructures in the crosslinkedcomposition, thereby yielding cross-linked polymer compounds with anexcellent mechanical strength. Furthermore, because of the distributionof the reactive silane groups, a high elasticity may be obtained aftercross-linking.

With respect to illustrating the above concepts in a resulting deviceconfiguration, FIG. 11 illustrates a lighting device 1100. Device 1100has a blue LED 1102 with a layer 1104 having a dispersion of quantumdots 1106 therein, in accordance with an embodiment of the presentinvention. Devices such as 1100 may be used to produce “cold” or “warm”white light. In one embodiment, the device 1100 has little to no wastedenergy since there is little to no emission in the IR regime. In aspecific such embodiment, the use of a layer having a composition with adispersion of quantum dots based on semiconductor hetero-structureshaving nano-crystalline cores composed of group I-III-VI materialstherein enables greater than approximately 40% lm/W gains versus the useof conventional phosphors. Increased efficacy may thus be achieved,meaning increased luminous efficacy based on lumens (perceived lightbrightness) per watt electrical power. Accordingly, down converterefficiency and spectral overlap may be improved with the use of adispersion of quantum dots to achieve efficiency gains in lighting anddisplay. In an additional embodiment, a conventional phosphor is alsoincluded in the composition, along with the dispersion of quantum dots1106.

Different approaches may be used to provide a quantum dot layer in alighting device. In an example, FIG. 12 illustrates a cross-sectionalview of a lighting device 1200 with a layer having a composition with adispersion of quantum dots therein, in accordance with an embodiment ofthe present invention. Referring to FIG. 12, a blue LED structure 1202includes a die 1204, such as an InGaN die, and electrodes 1206. The blueLED structure 1202 is disposed on a coating or supporting surface 1208and housed within a protective and/or reflective structure 1210. A layer1212 is formed over the blue LED structure 1202 and within theprotective and/or reflective structure 1210. The layer 1212 has acomposition including a dispersion of quantum dots or a combination of adispersion of quantum dots and conventional phosphors. Although notdepicted, the protective and/or reflective structure 1210 can beextended upwards, well above the matrix layer 1212, to provide a “cup”configuration.

In another example, FIG. 13 illustrates a cross-sectional view of alighting device 1300 with a layer having a composition with a dispersionof quantum dots therein, in accordance with another embodiment of thepresent invention. Referring to FIG. 13, the lighting device 1300includes a blue LED structure 1302. A quantum dot down converter screen1304 is positioned somewhat remotely from the blue LED structure 1302.The quantum dot down converter screen 1304 includes a layer with acomposition having a dispersion of quantum dots therein, e.g., ofvarying color, or a combination of a dispersion of quantum dots andconventional phosphors. In one embodiment, the device 1300 can be usedto generate white light, as depicted in FIG. 13.

In another example, FIG. 14 illustrates a cross-sectional view of alighting device 1400 with a layer having a composition with a dispersionof quantum dots therein, in accordance with another embodiment of thepresent invention. Referring to FIG. 14, the lighting device 1400includes a blue LED structure 1402 supported on a substrate 1404 whichmay house a portion of the electrical components of the blue LEDstructure 1402. A first conversion layer 1406 has a composition thatincludes a dispersion of red-light emitting quantum dots therein. Asecond conversion layer 1408 has a second composition that includes adispersion of quantum dots or green or yellow phosphors or a combinationthereof (e.g., yttrium aluminum garnet, YAG phosphors) therein.Optionally, a sealing layer 1410 may be formed over the secondconversion layer 1408, as depicted in FIG. 14. In one embodiment, thedevice 1400 can be used to generate white light.

In another example, FIG. 15 illustrates a cross-sectional view of alighting device 1500 with a layer having a composition with a dispersionof quantum dots therein, in accordance with another embodiment of thepresent invention. Referring to FIG. 15, the lighting device 1500includes a blue LED structure 1502 supported on a substrate 1504 whichmay house a portion of the electrical components of the blue LEDstructure 1502. A single conversion layer 1506 has a composition thatincludes a dispersion of red-light emitting quantum dots in combinationwith a dispersion of green quantum dots or green and/or yellow phosphorstherein. Optionally, a sealing layer 1510 may be formed over the singleconversion layer 1506, as depicted in FIG. 15. In one embodiment, thedevice 1500 can be used to generate white light.

In additional examples, FIGS. 16A-16C illustrate cross-sectional viewsof various configurations for lighting devices 1600A-1600C with a layerhaving a composition with a dispersion of quantum dots therein,respectively, in accordance with another embodiment of the presentinvention. Referring to FIGS. 16A-16C, the lighting devices 1600A-1600Ceach include a blue LED structure 1602 supported on a substrate 1604which may house a portion of the electrical components of the blue LEDstructure 1602. A conversion layer 1606A-1606C, respectively, has acomposition that includes a dispersion of one or more light-emittingcolor types of quantum dots therein. Referring to FIG. 1600Aspecifically, the conversion layer 1606A is disposed as a thin layeronly on the top surface of the blue LED structure 1602. Referring toFIG. 1600B specifically, the conversion layer 1606B is disposed as athin layer conformal with all exposed surfaces of the blue LED structure1602. Referring to FIG. 1600C specifically, the conversion layer 1606Cis disposed as a “bulb” only on the top surface of the blue LEDstructure 1602. In the above examples (e.g., FIGS. 11-15 and 16A-16C),although use with a blue LED is emphasized, it is to be understood thata layer having a composition with a dispersion of quantum dots thereincan be used with other light sources as well, including LEDs other thanblue LEDs.

In another aspect, synthetic approaches are provided for forming groupI-III-VI cores for hetero-structure particles, such as quantum dots.

In a first example, high quality copper indium sulfide (CIS) particleswere grown from a reaction using copper iodide, indium acetate, and in asolvent of dodecanethiol (DDT), which also serves as the sulfurprecursor. In this reaction, all components are mixed together at roomtemperature, degas sed, and heated to approximately 230 degrees Celsius.The particles nucleate and grow during this heating process, and it wasfound that the more rapidly the reaction is heated, the more uniform thecrystals appear to grow. FIG. 17 is a transmission electron microscopeimage 1700 of copper indium sulfide (CIS) nano-particles ofapproximately 3 nanometers in size, with a scale bar of 5 nanometers, inaccordance with an embodiment of the present invention. The CISnano-particles of image 1700 are suitable for use as emitting cores inhetero-structure based quantum dots.

In a second example, for the principle silver gallium sulfide (AGS)synthetic reaction, a system of reagents was chosen for general safehandling and compatibility with the CIS growth parameters. FIG. 18 is aschematic 1800 showing a general growth method for silver galliumsulfide (AGS) nano-particles, in accordance with an embodiment of thepresent invention. In one embodiment, using a Ag:Ga ratio of 1:2 yieldedan emission peak at approximately 550 nanometers, while using a Ag:Garatio of 1:4 yielded an emission peak at approximately 500 nanometers.

Thus, referring again to FIG. 18, in an embodiment, a method offabricating a semiconductor structure involves forming a first solutionincluding a gallium (Ga) source and a silver (Ag) source. The methodnext involves adding sulfur (S) to the first solution to form a secondsolution including the Ga source, the Ag source, and the sulfur. Thesecond solution is heated to form a plurality of silver gallium sulfide(AGS) nano-particles, e.g., for use as nano-crystalline cores in ahetero-structure quantum dot particle. In one embodiment, the pluralityof AGS nano-particles is formed to have a stoichiometry of approximatelyAgGaS₂.

Referring again to FIG. 18, in one such embodiment, the first solutionis formed by dissolving gallium acetylacetonate (ACAC) and silvernitrate (AgNO₃) in dodecanethiol (DDT) in the presence of a mixture ofcarboxylic acid. In a specific such embodiment, the method furtherinvolves degassing the first solution while heating the first solutionat a temperature of approximately 100 degrees Celsius. In a particularsuch embodiment, the method further involves, subsequent to thedegassing, heating the first solution to a temperature of approximately150 degrees Celsius under an atmosphere of argon (Ar). In oneembodiment, the first solution is formed by using a Ga source to Agsource ratio of at least approximately 1:2.

In another such embodiment, the second solution is formed by rapidlyinjecting the sulfur into the first solution. In one embodiment, themethod further involves heating the second solution to a temperature ofapproximately 250 degrees Celsius.

FIG. 19 is an X-ray diffraction (XRD) plot 1900 of intensity (counts) asa function of Two-Theta (degrees) for a sample of silver gallium sulfide(AGS) nano-particles fabricated according to methods described herein,in accordance with an embodiment of the present invention. Plot 1900appears to conclusively indicate that a form of AGS is in fact beinggrown in the chalcopyrite phase.

In an embodiment, 550 nanometer particles were grown in a 1:2 ratio ofAg:Ga, while particles having a 500 nanometer bandgap were grown using a1:4 ratio. FIG. 20 is a fluorescence spectrum 2000 of silver galliumsulfide (AGS) nano-particles grown with a 1:2 Ag:Ga precursor ratio, inaccordance with an embodiment of the present invention. The peak at 550nm, far redder than the predicted 470 nm, suggests that the crystalshave a stoichiometry other than AgGaS₂. By comparison, FIG. 21 is afluorescence spectrum 2100 of silver gallium sulfide (AGS)nano-particles grown with a 1:4 Ag:Ga precursor ratio, in accordancewith an embodiment of the present invention. The blue shifted peak at500 nm suggests that the crystals have a stoichiometry approachingcloser to the ideal AgGaS₂.

Thus, in an embodiment, the bandgap of AGS particles can be tuned by theAg:Ga ratio. Accordingly, AGS particles can be used as emitters,allowing for fine color tuning between 475-575 nm. In one embodiment, inorder to improve the emission of the AGS particles, ZnS is used as ashell material to rapidly protect the AGS surface. In one embodiment,the AGS particles are fabricated by batch processing. Other embodimentsinclude microwave assisted synthesis or continuous flow synthesis.

Thus, nano-crystalline core and nano-crystalline shell pairings havinggroup I-III-VI material nano-crystalline cores, and methods offabricating nano-crystalline core and nano-crystalline shell pairingshaving group I-III-VI material nano-crystalline cores, have beendisclosed.

What is claimed is:
 1. A semiconductor structure, comprising: anano-crystalline core comprising a group I-III-VI semiconductormaterial; and a nano-crystalline shell comprising a group I-III-VIsemiconductor material at least partially surrounding thenano-crystalline core, wherein the nano-crystallinecore/nano-crystalline shell pairing has a photoluminescence quantumyield (PLQY) of greater than 60%; and wherein the nano-crystallinecore/nano-crystalline shell pairing is a pairing selected from the groupconsisting of copper indium sulfide (CIS)/silver gallium sulfide(AgGaS₂), copper indium selenide (CISe)/AgGaS₂, copper gallium selenide(CuGaSe₂)/copper gallium sulfide (CuGaS₂), and CuGaSe₂/AgGaS₂.
 2. Thesemiconductor structure of claim 1, wherein the nano-crystalline core isan emitter having a direct, bulk band gap approximately in the range of1-2.5 eV.
 3. The semiconductor structure of claim 1, wherein thenano-crystalline core and nano-crystalline shell have a lattice mismatchof equal to or less than approximately 10%.
 4. The semiconductorstructure of claim 1, further comprising: a nano-crystalline outer shellcomprising a third, different, semiconductor material at least partiallysurrounding the nano-crystalline shell.
 5. The semiconductor structureof claim 4, wherein the third semiconductor material is zinc sulfide(ZnS).
 6. The semiconductor structure of claim 1, further comprising: acompositional transition layer disposed between, and in contact with,the nano-crystalline core and nano-crystalline shell, the compositionaltransition layer having a composition intermediate to thenano-crystalline core semiconductor material and the nano-crystallineshell semiconductor material.
 7. The semiconductor structure of claim 6,wherein the compositional transition layer is an alloyed layercomprising a mixture of the nano-crystalline core semiconductor materialand the nano-crystalline shell semiconductor material.
 8. Thesemiconductor structure of claim 6, wherein the compositional transitionlayer is a graded layer comprising a compositional gradient of thenano-crystalline core semiconductor material proximate to thenano-crystalline core through to the nano-crystalline shellsemiconductor material proximate to the nano-crystalline shell.
 9. Thesemiconductor structure of claim 1, wherein the nano-crystalline core isanisotropic nano-crystalline core having an aspect ratio between, butnot including, 1.0 and 2.0.
 10. The semiconductor structure of claim 9,wherein the nano-crystalline shell is an anisotropic nano-crystallineshell having an aspect ratio approximately in the range of 2-6.
 11. Thesemiconductor structure of claim 1, further comprising: an insulatorcoating surrounding and encapsulating the nano-crystallinecore/nano-crystalline shell pairing.
 12. The semiconductor structure ofclaim 11, wherein the insulator coating comprises an amorphous materialselected from the group consisting of silica (SiO_(x)), titanium oxide(TiO_(x)), zirconium oxide (ZrO_(x)), alumina (AlO_(x)), and hafnia(HfO_(x)).
 13. The semiconductor structure of claim 1, wherein thenano-crystalline shell completely surrounds the nano-crystalline core.14. The semiconductor structure of claim 1, wherein the nano-crystallineshell only partially surrounds the nano-crystalline core, exposing aportion of the nano-crystalline core.
 15. The semiconductor structure ofclaim 1, wherein the nano-crystalline core is disposed in an asymmetricorientation with respect to the nano-crystalline shell.
 16. Thesemiconductor structure of claim 1, wherein the nano-crystalline coreand nano-crystalline shell form a quantum dot.
 17. The semiconductorstructure of claim 16, wherein the quantum dot is a down-convertingquantum dot.
 18. A semiconductor structure, comprising: anano-crystalline core comprising a group I-III-VI semiconductormaterial; and a nano-crystalline shell comprising a group I-III-VI,semiconductor material at least partially surrounding thenano-crystalline core, wherein the nano-crystallinecore/nano-crystalline shell pairing is a Type I hetero-structure; andwherein the nano-crystalline core/nano-crystalline shell pairing is apairing selected from the group consisting of copper indium sulfide(CIS)/silver gallium sulfide (AgGaS₂), copper indium selenide(CISe)/AgGaS₂, copper gallium selenide (CuGaSe₂)/copper gallium sulfide(CuGaS₂), and CuGaSe₂/AgGaS₂.
 19. The semiconductor structure of claim18, wherein the nano-crystalline core is an emitter having a direct,bulk band gap approximately in the range of 1-2.5 eV.
 20. Thesemiconductor structure of claim 18, wherein the nano-crystalline coreand nano-crystalline shell have a lattice mismatch of less thanapproximately 4%.
 21. The semiconductor structure of claim 18, furthercomprising: a nano-crystalline outer shell comprising a third,different, semiconductor material at least partially surrounding thenano-crystalline shell.
 22. The semiconductor structure of claim 21,wherein the third semiconductor material is zinc sulfide (ZnS).
 23. Thesemiconductor structure of claim 18, further comprising: a compositionaltransition layer disposed between, and in contact with, thenano-crystalline core and nano-crystalline shell, the compositionaltransition layer having a composition intermediate to thenano-crystalline core semiconductor material and the nano-crystallineshell semiconductor material.
 24. The semiconductor structure of claim23, wherein the compositional transition layer is an alloyed layercomprising a mixture of the nano-crystalline core semiconductor materialand the nano-crystalline shell semiconductor material.
 25. Thesemiconductor structure of claim 23, wherein the compositionaltransition layer is a graded layer comprising a compositional gradientof the nano-crystalline core semiconductor material proximate to thenano-crystalline core through to the nano-crystalline shellsemiconductor material proximate to the nano-crystalline shell.
 26. Thesemiconductor structure of claim 18, wherein the nano-crystalline coreis anisotropic nano-crystalline core having an aspect ratio between, butnot including, 1.0 and 2.0.
 27. The semiconductor structure of claim 26,wherein the nano-crystalline shell is an anisotropic nano-crystallineshell having an aspect ratio approximately in the range of 2-6.
 28. Thesemiconductor structure of claim 18, further comprising: an insulatorcoating surrounding and encapsulating the nano-crystallinecore/nano-crystalline shell pairing.
 29. The semiconductor structure ofclaim 28, wherein the insulator coating comprises an amorphous materialselected from the group consisting of silica (SiO_(x)), titanium oxide(TiO_(x)), zirconium oxide (ZrO_(x)), alumina (AlO_(x)), and hafnia(HfO_(x)).
 30. The semiconductor structure of claim 18, wherein thenano-crystalline shell completely surrounds the nano-crystalline core.31. The semiconductor structure of claim 18, wherein thenano-crystalline shell only partially surrounds the nano-crystallinecore, exposing a portion of the nano-crystalline core.
 32. Thesemiconductor structure of claim 18, wherein the nano-crystalline coreis disposed in an asymmetric orientation with respect to thenano-crystalline shell.
 33. The semiconductor structure of claim 18,wherein the nano-crystalline core and nano-crystalline shell form aquantum dot.
 34. The semiconductor structure of claim 33, wherein thequantum dot is a down-converting quantum dot.
 35. A composite,comprising: a matrix material; and a plurality of semiconductorstructures embedded in the matrix material, each semiconductor structurecomprising: a nano-crystalline core comprising a group I-III-VIsemiconductor material; a nano-crystalline shell comprising a second,different, semiconductor material at least partially surrounding thenano-crystalline core, wherein the nano-crystallinecore/nano-crystalline shell pairing has a photoluminescence quantumyield (PLQY) of greater than 60%; and an amorphous insulator coatingsurrounding and encapsulating the nano-crystalline core/nano-crystallineshell pairing wherein one or more of the semiconductor structuresfurther comprises a coupling agent covalently bonded to an outer surfaceof the amorphous insulator coating.
 36. The composite of claim 35,wherein each of the plurality of semiconductor structures iscross-linked with, polarity bound by, or tethered to the matrixmaterial.
 37. The composite of claim 35, wherein each of the pluralityof semiconductor structures is bound to the matrix material by acovalent, dative, or ionic bond.
 38. The composite of claim 35, wherein,for each of the plurality of semiconductor structures, the secondsemiconductor material is a group I-III-VI material.
 39. The compositeof claim 38, wherein, for each of the plurality of semiconductorstructures, the nano-crystalline core/nano-crystalline shell pairing isa pairing selected from the group consisting of copper indium sulfide(CIS)/silver gallium sulfide (AgGaS₂), copper indium selenide(CISe)/AgGaS₂, copper gallium selenide (CuGaSe₂)/copper gallium sulfide(CuGaS₂), and CuGaSe₂/AgGaS₂.
 40. A composite, comprising: a matrixmaterial; and a plurality of semiconductor structures embedded in thematrix material, each semiconductor structure comprising: anano-crystalline core comprising a group I-III-VI semiconductormaterial; a nano-crystalline shell comprising a second, different,semiconductor material at least partially surrounding thenano-crystalline core, wherein the nano-crystallinecore/nano-crystalline shell pairing is a Type I hetero-structure; and anamorphous insulator coating surrounding and encapsulating thenano-crystalline core/nano-crystalline shell pairing wherein one or moreof the semiconductor structures further comprises a coupling agentcovalently bonded to an outer surface of the amorphous insulatorcoating.
 41. The composite of claim 40, wherein each of the plurality ofsemiconductor structures is cross-linked with, polarity bound by, ortethered to the matrix material.
 42. The composite of claim 40, whereineach of the plurality of semiconductor structures is bound to the matrixmaterial by a covalent, dative, or ionic bond.
 43. The composite ofclaim 40, wherein, for each of the plurality of semiconductorstructures, the second semiconductor material is a group I-III-VImaterial.
 44. The composite of claim 43, wherein, for each of theplurality of semiconductor structures, the nano-crystallinecore/nano-crystalline shell pairing is a pairing selected from the groupconsisting of copper indium sulfide (CIS)/silver gallium sulfide(AgGaS₂), copper indium selenide (CISe)/AgGaS₂, copper gallium selenide(CuGaSe₂)/copper gallium sulfide (CuGaS₂), and CuGaSe₂/AgGaS₂.